Although append currently returns the edited list, this behavior is unstable
and will change in future releases to
undefined, to bring its implementation in
line with what is in the Sentinel Language
Specification.

To explain why this is not recommended and why it will change in the future,
let's look at the use of the function in the context of a
rule. If append returned the list value as a
result, the following policy would depend on the order in which rules are
referenced:

The side effects of this behavior introduce complexity in policies that makes
them prone to logical errors which are difficult to track. As a result,
functions like these should return undefined and modify values in-place.